Osteoid osteoma is a benign, bone-forming lesion, seen most commonly in the diaphysis of long bones such as femur and tibia. Most commonly osteoid osteoma is seen in the age group between 10 and 30 years with male predominance ^[1]. It accounts for 10–14% of all benign and 2–3% of all primary bone tumors ^[2]. It is not very uncommon to have not diagnosed the condition in spite of having symptoms for a very long time in a young active individual. Nocturnal pain is considered to be the predominant symptom and usually described as dull aching, constant, intense pain which improves by salicylates and non-steroidal anti-inflammatory drugs ^[2]. Typically, osteoid osteoma consists of radiolucent nidus <2 cm in diameter and a rim of reactive osteosclerosis on radiological investigations ^[2]. Generally, osteoid osteomas are managed conservatively, failing which surgical treatment is considered. Recently, minimally invasive techniques like radiofrequency ablation, laser ablation, and cryoablation have replaced open en bloc resection due to its potential complications ^[2]. Occurrence of osteoid osteoma in patella is considered to be very rare with an incidence of 5% of all patellar tumors ^[3]. Long-standing anterior knee pain is considered to be the most common presenting feature in patients with patellar osteoid osteoma ^[1]. Due to rarity and atypical presentation of osteoid osteoma in patella, it results in misdiagnosis and inappropriate management.

A 21-year-old male presented to our outpatient department with a history of insidious onset of right anterior knee pain for the past 4 years. Intensity of pain increased over a period of time due to which patient had difficulty in participating in sports activities, using public transport, and riding 2 wheeler. Patient experienced pain throughout the day with no significant diurnal variations. There was no history of previous trauma. Since the onset of pain, the patient had numerous consultations and was treated with analgesics. The pain subsided briefly following medications but relapsed after a few weeks. On examination, patient had minimal tenderness over patella, the knee range of movements was full and associated with pain, and quadriceps muscle wasting was noted. On plain radiograph of the right knee joint, no significant findings were seen. Magnetic resonance imaging (MRI) was suggestive of well-defined oval T1-hypointense and T2-hyperintense lesion in the subchondral aspect of the patella with mild surrounding marrow edema-likely osteoid osteoma (Fig. 1). CT scan showed a well-defined lucent lesion measuring 4 × 5 × 5 mm with a sclerotic rim along the subchondral aspect of posterior articular surface, confirming the diagnosis osteoid osteoma (Fig. 2).

Percutaneous CT-guided radiofrequency ablation was planned for the patient. Under general anesthesia lesion was localized on multidetector CT.3.5 mm drill bit was used to create a tract into the lesion from the ventral surface of patella under CT guidance. Care was taken to ensure patellar articular cartilage is not damaged at the time of procedure. Radiofrequency ablation probe with active tip of 9 mm was placed in the lesion (Fig. 3). Ablation was carried out beginning with 2 Watts and increased at the rate of 1 Watt per minute till maximum impedance is achieved. No procedure-related complications were noted. Postoperatively, patient was administered analgesics for 1 week and was made to walk full weight bearing. One year following radiofrequency ablation of the lesion patient is pain-free and having full range of movements.

Osteoid osteoma is a benign, usually solitary bone-forming tumor accounting for 10–14% of all benign- and 2–3% of all primary-bone tumors ^[2]. About 90% of cases are seen in younger individuals before the age of 25 years with male to female ratio of 2:1 ^[4]. Clinically osteoid osteoma presents as severe local pain which worsens at night and usually relieved by NSAIDs or aspirin. Radiologically they present as radiolucent nidus sized usually between 1.5 and 2 cm with a reactive sclerotic rim. However, the diagnostic efficacy of the plain radiography is less, and chances of misdiagnosing increases, more so with the intra-articular lesions. Thin section CT analysis provides a better chance to pick up the lesion and to make a diagnosis. Tc-MDP bone scan also be used as a diagnostic tool with double-density sign presentation ^[5]. Although MRI can pick up the nidus, reactive marrow edema can obscure the nidus, thereby making diagnosis difficult at times ^[5]. The average duration of time to diagnose intra-articular osteoid osteoma ranges from 4 to 5 years according to literature ^[6]. In our case report, the time to diagnosis was 4 years attributed to lack of early clinical suspicion and conservative management with medications considering its rarity.

Spontaneous regression of osteoid osteoma has been noted but it requires strict monitoring and prolonged medications in the form NSAIDs or aspirin. Surgical management of osteoid osteoma is planned when the medical management fails. Open resection was considered to be the choice of management earlier, however, they carry the disadvantages of an extensive surgical approach. Vallianatos et al. reported a complication of Patella Baja in their study after an open surgical excision of patellar osteoid osteoma possibly due to patellar tendon scarring ^[7]. Bavaneh et al. used mosaicplasty technique following open resection for the cartilage defect with no recurrence after a mean follow-up of 31 months ^[8]. Chillemi et al. proposed CT-guided drilling of nidus with good results ^[4] while Franceschi et al. described CT-guided arthroscopic resection of subchondral patellar osteoid osteoma ^[9]. Our patient was a young and active adult with intractable knee pain, apprehensive of an open surgical procedure and recurrence. Minimally invasive techniques include laser photocoagulation, radiofrequency ablation, percutaneous drilling, and ethanol injection. Interstitial Laser Photocoagulation is a safe and effective procedure having a few advantages like non-necessity of a neutral electrode, no current flow through the patient’s body, and no interaction with pacemakers ^[10]. Percutaneous Drilling and ethanol injection for osteoid osteomas studied by Gerhard Adam et al. was cost-effective but carried the risk of painful necrosis due to extravasation of excess ethanol ^[11]. Considering all of the above and the availability of a CT-guided radiofrequency ablation at our setup, we decided to go ahead with the same. CT-guided radiofrequency ablation is minimally invasive and performed under local anesthesia giving the benefit of accurate lesion localization, shorter hospital stay, and early rehabilitation. The success rate of Radiofrequency ablation is reported to be as high as 95% ^[12]. However, there might be a few disadvantages of CT-guided ablation such as high cost, radiation exposure, and risk of articular cartilage damage.

Occurrence of patellar osteoid osteomas is very rare and its atypical presentation can lead to misdiagnosis and delay in the treatment. Most commonly it is misdiagnosed to be as chondromalacia patella, osteochondritis dissecans, jumper’s knee, or patellofemoral dysplasia. A high degree of suspicion is required in patients having chronic anterior knee pain with atypical features such as night pain, rest pain, restricted knee range of movements, and quadriceps wasting to evaluate further for the possibility of patellar osteoid osteoma.

Although Patellar Osteoid Osteoma is a rare cause of Chronic Anterior Knee Pain, the possibility of it should be kept in mind during the course of evaluation. In cases with chronic non-remitting anterior knee pain, a high degree of clinical suspicion helps in the accurate diagnosis of Patellar Osteoid Osteoma. Radiofrequency ablation is a novel and minimally invasive procedure for the treatment of Patellar Osteoid Osteoma which can help in resolution of anterior knee pain.